Manufacturing process for high-purity phosphors having utility in field emission displays

ABSTRACT

A process is provided for manufacturing high-purity phosphors having utility in field emission displays. The high-purity phosphor is a host lattice infiltrated by a dopant that activates luminescent properties therein. The lattice and dopant are initially milled together to reduce their average particle size while simultaneously achieving complete mixing between the lattice and the dopant. The resulting mixture is maintained free of a flux or substantially any other treatment agent capable of contaminating the phosphor and placed in a heating vessel formed from a substantially impervious contaminant-free material. The mixture is heated to a high temperature effectuating thorough infiltration of the dopant into the lattice structure. The use of an impervious contaminant-free heating vessel and the exclusion of flux or other treatment agents from the mixture avoids undesirable contamination and undue particle size growth of the phosphor product during the manufacture thereof. Accordingly, product is a high-purity phosphor having a small average particle size, yet exhibiting sufficient luminescent efficiencies for utility in field emission displays as a luminescent coating for the anode screen.

This invention was made with Government support under Contract No.DABT6393-O-0025 awarded by Advanced Research Projects Agency (ARPA). TheGovernment has certain rights in this invention.

TECHNICAL FIELD

The present invention relates generally to the manufacture of phosphors,and more particularly to a process for manufacturing high-purityphosphors used in field emission displays.

BACKGROUND OF THE INVENTION

Luminescent materials, termed phosphors, have general utility in a broadrange of lighting and display applications. A phenomenon common to allsuch applications is excitation of the phosphors in accordance with anyone of a number of techniques known in the art, causing the phosphors toemit light. Known excitation techniques include exposing the phosphorsto emissions from an external energy source. The emissions can be in theform of electrons, ultra violet, x-rays, or gamma rays, to name a few.Hence, this excitation phenomenon is apparent in virtually all types ofconventional, phosphor-containing, host lighting or display fixtures.Among such conventional fixtures are fluorescent tubes, cathode raytubes, liquid crystal displays, gas discharge plasma displays, vacuumfluorescent displays, and field emission displays.

Cathode ray tubes are typical of luminescent displays employing electronemissions as the excitation means for the phosphors. Such displays havean anode panel coated with phosphors that are selectively excited byelectrons directed toward the phosphors from an adjacentelectron-supplying cathode. The excited phosphors emit light, therebycreating a desired image visible to the viewer on the screen of thedisplay. Phosphors having utility for display applications typicallycomprise a host lattice impregnated with a quantity of a dopant thatactivates luminescent properties in the resulting composition. Thephosphors are conventionally manufactured by selecting the host latticeand dopant from among well-known materials. The selected lattice anddopant are mixed together and milled to a relatively uniform particlesize distribution. A typical average particle size for the mixture is onthe order of about 10 microns because it is believed that suchrelatively large particle sizes contribute to the luminescent efficiencyof the resulting phosphor product. A flux is also generally added to themixture to facilitate subsequent heat treatment thereof. Fluxes havingutility in the preparation of phosphors are characterized as materialshaving a relatively low melting point typically about 1000° C. or lessthat promote infiltration of the dopant into the lattice structure whenheated. Conventional fluxes include ammonium compounds, such as ammoniumchloride, and compounds combining Group I A or II A elements and GroupVI A or VII A elements, such as alkali metal halides and alkaline earthmetal halides. Other agents facilitating heat treatment of the latticeand dopant mixture can also be combined with the mixture such as sulfurwhich serves as an antioxidant.

The composition comprising the host lattice, dopant, and flux, as wellas any other selected treatment agents, is placed in a crucible formedfrom a refractory material, such as silica or alumina, and heated abovethe melting point of the flux to effectuate infiltration of the dopantinto the host lattice. The presence of the flux, however, tends toinduce excessive growth of the lattice particles during heat treatment.Consequently, the heat treated composition may be remilled followingheat treatment to restore it to its original pretreatment particle size.Unfortunately, remilling the heat treated particles can negativelyimpact the luminescent efficiency of the resulting phosphor product byexposing surfaces of the phosphor product having relatively low dopantconcentrations. In any case, a final step in the manufacture ofphosphors is removal of the flux from the particles by means such aswater or acid washing to obtain the desired phosphor product.

Although the above-described prior art process produces phosphors ofadequate purity for many conventional display applications includingcathode ray tubes, it has been found that present-day field emissiondisplay applications require phosphors of greater purity than thoseproduced by such prior art processes. Specifically, it has been Foundthat residual quantities of flux unduly contaminate phosphor productsmanufactured in accordance with prior art processes even after washingthe product. Contaminants retained by the phosphors from the flux,namely Group I A or II A cations, are often at times generallyincompatible with silicon structures employed in the displays. Moreparticularly, such contaminants can cause failure of field emissiondisplays because the emitter tips that serve as the cathodes of a fieldemission display are extremely sensitive to contamination. Thepositively charged Group I A or II A cations are highly mobile in theevacuated environment of field emission displays. Group I A and II Acations readily migrate the relatively short distance from the anodeplate to the cathodic emitter tips. An excessive accumulation of suchcations on the emitter tips causes irreparable damage thereto.Refractory crucibles can likewise contribute Group I A or II Acontaminants to the phosphor product due to their relative porosity thatretains such contaminants and undesirably releases them into thephosphor product when heated.

As such, a need exists for a high-purity phosphor having specificutility to field emission display applications. Accordingly, it is anobject of the present invention to provide a process for manufacturing ahigh-purity phosphor that satisfies the performance demands of fieldemission displays. More particularly, it is an object of the presentinvention to provide a process for manufacturing a high-purity phosphorthat does not require a flux or any other treatment agent during heattreatment of the lattice and dopant. It is another object of the presentinvention to provide a process for manufacturing a high-purity phosphor,wherein the product is substantially free of contaminants from fluxes,other treatment agents, or process vessels which could diminish theoperability of cathodic emitter tips employed with the phosphors in afield emission display. It is yet another object of the presentinvention to provide a process for manufacturing a high-purity phosphorhaving a relatively small particle size, yet having an acceptableluminescent efficiency. It is still another object of the presentinvention to provide a process for manufacturing a high-purity phosphor,wherein the lattice and dopant are heat treated at a relatively hightemperature without substantially increasing the particle size thereof.It is a further object of the present invention to provide a process formanufacturing a high-purity phosphor, wherein the dopant is welldistributed throughout the host lattice. These objects and others areaccomplished in accordance with the invention described hereafter.

SUMMARY OF THE INVENTION

The present invention is a process for manufacturing high-purityphosphors having utility in luminescent displays, and specifically fieldemission displays. The phosphor produced by the present processcomprises a host lattice infiltrated by a dopant that activatesluminescent properties in the resulting composition. In accordance withthe process, a conventional lattice is initially provided in particulateform that is capable of hosting a selected dopant also provided inparticulate form. The lattice and dopant particles are processedtogether into a fine powder, substantially reducing their averageparticle size while simultaneously achieving complete mixing between thelattice and the dopant. The resulting mixture is maintained free of aflux or substantially any other treatment agent capable of contaminatingthe phosphor product with Group I A or II A contaminants and placed in aheating vessel formed from a substantially impervious contaminant-freematerial. The vessel and its contents are heated to a relatively hightemperature effectuating thorough infiltration of the dopant into thelattice structure.

The use of an impervious contaminant-free heating vessel and theexclusion of flux or other treatment agents from the mixture avoidsundesirable contamination and undue particle size growth of the phosphorproduct during the manufacture thereof. Accordingly, the present processdesirably produces relatively high-purity phosphors having a relativelysmall average particle size, yet exhibiting sufficient luminescentefficiencies for utility in field emission display applications. Theprocess of the present invention will be further understood from thedrawings and the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical representation of the particle size distributionsof precursor materials before and after grinding in accordance with thepresent invention.

FIG. 2 is a graphical representation of the particle size distributionof a phosphor product produced in accordance with the present invention.

FIG. 3 is a graphical representation of the particle size distributionof a commercial grade phosphor produced in accordance with conventionalprior art methods.

DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention relates to a process for manufacturing a phosphorfrom precursor materials, wherein the precursor materials are heattreated at a relatively high temperature in a substantiallycontaminant-free environment absent fluxes or othercontaminant-generating materials to achieve a high-purity phosphorproduct having specific utility in field emission displays. Ahigh-purity phosphor is defined herein as a luminescent compositionhaving at least one host lattice and at least one dopant impregnatedtherein, but substantially free of any contaminants capable of causingfailure or otherwise disrupting operation of the cathode in aluminescent display. In particular, the high-purity phosphor issubstantially free of any cationic contaminants, and more particularlysubstantially free of any Group I A or II A cations, contributed byfluxes or other treatment agents that are capable of damaging thecathodic emitter tips of a field emission display. The term"substantially free" relates to species having no detectableconcentration within the high-purity phosphor or having a concentrationbelow about one part per million within the high-purity phosphor.

The precursor materials of the phosphor consist of the one or moreselected host lattices and the one or more selected dopants prior to theperformance of any treatment steps hereunder. It is further noted thatthe selected precursor materials are preferably provided in a solidparticulate form. Compositions of host lattices having utility in thepresent invention are well known to the skilled artisan and include suchspecies as silicates, aluminates, oxides, garnets, gallates, vandates,tungstates, phosphates, pyrophosphates, fluorides, oxysulfides, andmixtures thereof. Preferred compositions of the host lattice are oxidesof yttrium or gallates. Gallates are binary species formed by combiningoxides of gallium and oxides of one or more selected metals, such aszinc, in stoichiometric amounts. An exemplary gallate is (Zn, Ga)_(x)O_(y). Compositions of dopants having utility in the present inventionare likewise well known to the skilled artisan and such dopants arealternately termed activators. Useful dopants include one or moreselected transition elements, and in particular one or more selectedlanthanides and/or one or more selected transition metals, such aseuropium, terbium, cerium, manganese, copper, aluminum, gold, silver,and mixtures thereof. Among the preferred dopants are europium, cerium,and terbium. The precursor materials preferably consist of a unitaryhost lattice and a unitary dopant, a binary host lattice and a unitarydopant, or a unitary host lattice and a binary dopant. Phosphorsproduced from a unitary host lattice and a unitary dopant are termedbinary phosphors, while phosphors produced from a binary host latticeand a unitary dopant or from a unitary host lattice and a binary dopantare termed ternary phosphors.

As can be appreciated by the skilled artisan, an important criterium forselection of the specific precursor materials is the desired color oflight to be emitted by the resulting phosphor manufactured in accordancewith the present process. It is well understood that specific phosphorsemit either red, blue or green light.

Upon selection of the precursor materials in accordance with theabove-recited criteria, the precursor materials, i.e., the lattice anddopant, are placed together in a processing vessel substantially free ofany other active species or of any contaminantgenerating species toobtain a reduced uniform particle size distribution thereof and toachieve complete mixing between the lattice and dopant particles. Thelattice and dopant are added to the processing vessel in relativeamounts such that the dopant typically comprises from about 0.1% toabout 10.0% by weight of the precursor materials retained in the vessel,the remaining weight percentage of the precursor materials being thehost lattice. The preferred processing vessel of this step is a particlesize reduction vessel, and more particularly a milling vessel utilizedin a mill such as a McCrone mill, a Fritsch planetary mill, or aconventional ball mill. Most preferred among these mills arereciprocating mills, such as the McCrone mill. Although, as noted above,the precursor materials are maintained substantially free of any otheractive species or of any contaminant-generating species during thisstep, an inert, non-contaminating liquid medium is often combined withthe precursor materials to form a slurry within the milling vessel,thereby enhancing the effectiveness of the milling step. In any case,the precursor materials are sufficiently milled in the selected mill toobtain a mixture of the lattice and dopant having an average particlesize less than about 2 microns and preferably less than about 1 micron.If a liquid medium is employed in the milling step, the slurrycontaining the milled mixture of precursor materials is thoroughly driedto drive substantially all of the liquid medium therefrom.

The next step in the sequence of the process is to heat treat the milledmixture of precursor materials, thereby thoroughly impregnating thedopant within the host lattice structure. The high purity of theeventual phosphor product is insured by heat treating the milled mixtureof precursor materials in the absence of any fluxes or other agents thatare capable of introducing contaminants, and particularly Group I A orGroup II A contaminants, into the mixture. Accordingly, the compositionbeing heat treated is essentially limited to the milled mixturecontaining the host lattice and dopant and is substantially free of anycontaminants that could impair operation of the cathode when thephosphor product is employed in a luminescent display. The high purityof the phosphor product is further insured by employing a relativelypure, substantially impervious, nonporous crucible as the containmentvessel for the milled mixture during heat treatment thereof such thatprocess equipment contacting the heated mixture does not introduce anycontaminants therein. Crucibles satisfying these criteria are formed,for example, from platinum or iridium, of which platinum is preferred.

Heat treatment of the milled mixture is performed by placing thecrucible and its contents in a conventional heating means such as anoven or a kiln. The milled mixture is heated therein to a temperaturebetween about 1200° C. and 2000° C., preferably between about 1400° C.and 1800° C., and more preferably to about 1600° C. The mixture istypically maintained in the heating means within the above-prescribedtemperature range for a time period between about 0.5 and about 6.0hours or more depending on the specific composition of the mixture. Theatmosphere of the heating means is air or a reducing atmosphere such ascarbon monoxide or hydrogen gas. Sulfur-containing atmospheres arepreferably avoided, as exposure to sulfur-containing gases may bedetrimental to the resulting phosphor product. Under these conditions,the dopant thoroughly permeates the host lattice structure, but due tothe absence of a flux, the product does not exhibit substantial particlesize growth. The average particle size of the product after heattreatment is generally no greater than 100% larger than the averageparticle size of the mixture prior to heat treatment, and preferably nogreater than 50%. Consequently, manufacture of the phosphor product isessentially complete upon performance of the heat treatment, obviatingthe need to remill or wash the product.

The resulting high-purity phosphor product has general utility forconventional luminescent lighting and display applications, enhancingthe performance thereof. The high-purity phosphor product, however, hasspecific utility in field emission display applications. Field emissiondisplay devices employ cold cathode emitters in the form of a pluralityof emitter tips that direct electron emissions in an evacuatedenvironment toward an adjacent anode screen in relatively closeproximity thereto. The anode screen has a phosphor coating applied thatis excited by electrons from the emitter tips, thereby selectivelyilluminating the screen. Numerous embodiments of such field emissiondisplay devices are known in the art, for example, as disclosed by U.S.Pat. Nos. 5,229,331 and 5,232,549, which are incorporated herein byreference. Accordingly, a high-purity phosphor produced in the manner ofthe present invention is substituted for those phosphors disclosed inthe prior art as the coating on the anode screen of the field emissiondisplay. The present high-purity phosphors have been found to enhanceboth the performance and the longevity of field emission displays inwhich the phosphors are employed as a coating on the anode screenbecause the high-purity phosphors do not contain contaminants thatadversely affect the emitter tips diminishing the expected usefullifetime of the display device.

The following examples demonstrate the practice and utility of thepresent invention, but are not to be construed as limiting the scopethereof.

EXAMPLE 1

A pair of stock precursor materials is selected consisting of a Y₂ O₃host lattice and a Eu₂ O₃ dopant, both in particulate form. Theprecursor materials are placed in a particle size analyzer and themajority of the particles are determined to exceed 4 microns in size. Inparticular, the Y₂ O₃ host lattice has a measured average particle sizeof 5.336 microns and the EU₂ O₃ dopant has a measured average particlesize of 4.031 microns. Curves indicating the particle size distributionof the host lattice and the dopant, respectively, are displayed togetherin FIG. 1. The precursor materials are then placed in a liquid medium toform a slurry and milled together in a McCrone mill. The averageparticle size of the resulting milled mixture of precursor materials is1.615 microns. A curve indicating the particle size distribution of themilled mixture of precursor materials is displayed in FIG. 1 adjacent tothe particle size distribution curves of the host lattice and thedopant. FIG. 1 demonstrates that the precursor materials undergosubstantial particle size reduction when milled in accordance with theprocess of the present invention.

EXAMPLE 2

A Y₂ O₃ : Eu phosphor product is produced from a Y₂ O₃ host lattice anda Eu₂ O₃ dopant by initially milling the precursor materials insubstantially the same manner as Example 1. The milled mixture ofprecursor materials is then dried overnight at a temperature of 90° C.The dried milled mixture is heat treated by firing it in the absence ofa flux or any other treatment agents for a period of 2 hours at atemperature of 1550° C to complete formation of the desired Y₂ O₃ : Euphosphor product. Size analysis of the resulting phosphor productindicates that it has an average particle size of 1.810 microns. A curveindicating the particle size distribution of the phosphor product isdisplayed in FIG. 2. For comparison purposes, a size analysis isconducted on a commercial grade Y₂ O₃ : Eu phosphor produced inaccordance with conventional prior art methods indicating that thecommercial grade phosphor has an average particle size of 7.090 microns.FIG. 3 shows the particle size distribution of the commercial gradephosphor. Comparison of FIGS. 2 and 3 demonstrates that the phosphorproduct produced in accordance with the present invention achieves asubstantially reduced particle size relative to conventional phosphorswithout requiring post heat-treatment milling of the product.Compositional analysis of the phosphor product of FIG. 2 also shows thatit is substantially free of any contaminants. Accordingly, the phosphorproduct is suitably pure for use as an effective anode screen coating ina conventional field emission display.

While forgoing preferred embodiments of the invention have beendescribed and shown, it is understood that alternatives andmodifications, such as those suggested and others, may be made theretoand fall within the scope of the invention.

We claim:
 1. A process for preparing a high-purity phosphor havingutility as a luminescent in a field emission display, said processcomprising:providing a host lattice starting material and a dopantstarting material; combining said host lattice starting material andsaid dopant starting material in a precursor mixture having an initialaverage particle size and substantially free of anycontaminant-generating components containing a Group IA ion or a GroupIIA ion; processing said precursor mixture to obtain a sized precursormixture having an average precursor particle size less than about 2microns; placing said sized precursor mixture in a heat treatment vesselformed from platinum; and heating said sized precursor mixture in saidheat treatment vessel to a temperature between about 1200° C. and 2000°C. for a time sufficient to infiltrate said dopant starting materialinto said host lattice starting material, thereby producing ahigh-purity phosphor.
 2. A process for preparing a high-purity phosphoras recited in claim 1 wherein said host lattice starting material isselected from the group consisting of silicates, aluminates, oxides,gallates, vandates, tungstates, phosphates, fluorides, oxysulfides, andmixtures thereof.
 3. A process for preparing a high-purity phosphor asrecited in claim 1 wherein said dopant starting material is selectedfrom the group consisting of lanthanides, transition metals and mixturesthereof.
 4. A process for preparing a high-purity phosphor as recited inclaim 1 wherein said high-purity phosphor is a binary or a ternaryphosphor.
 5. A process for preparing a high-purity phosphor as recitedin claim 1 wherein said sized precursor mixture is heated to atemperature between about 1400° C. and 1800° C.
 6. A process forpreparing a high-purity phosphor as recited in claim 1 wherein saidaverage precursor particle size is less than about 1 micron.
 7. Aprocess for preparing a high-purity phosphor as recited in claim 1wherein said phosphor has an average product particle size no greaterthan about 100% larger than said average precursor particle size.
 8. Aprocess for preparing a high-purity phosphor as recited in claim 1wherein said phosphor has an average product particle size no greaterthan about 50% larger than said average precursor particle size.
 9. Aprocess for preparing a high-purity phosphor as recited in claim 1wherein said components are fluxes.
 10. A process for preparing ahigh-purity phosphor as recited in claim 1 wherein said precursormixture is processed to obtain said sized precursor mixture by millingsaid precursor mixture in a reciprocating mill, thereby reducing saidinitial average particle size of said precursor mixture to said averageprecursor particle size of said sized precursor mixture.
 11. A processfor manufacturing an anode of a field emission display having ahigh-purity phosphor coating, said process comprising:providing a hostlattice starting material and a dopant starting material; combining saidhost lattice starting material and said dopant starting material in aprecursor mixture having an initial average particle size andsubstantially free of any contaminant-generating components containing aGroup IA ion or a Group IIA ion; processing said precursor mixture toobtain a sized precursor mixture having an average precursor particlesize less than about 2 microns; placing said sized precursor mixture ina heat treatment vessel formed from platinum; heating said sizedprecursor mixture in said heat treatment vessel to a temperature betweenabout 1200° C. and 2000° C. for a time sufficient to infiltrate saiddopant starting material into said host lattice starting material,thereby producing a high-purity phosphor; and applying said high-purityphosphor to an anode screen of a field emission display.
 12. A processfor preparing a high-purity phosphor as recited in claim 11 wherein saidhost lattice starting material is selected from the group consisting ofsilicates, aluminates, oxides, gallates, vandates, tungstates,phosphates, fluorides, oxysulfides, and mixtures thereof.
 13. A processfor preparing a high-purity phosphor as recited in claim 11 wherein saiddopant starting material is selected from the group consisting oflanthanides, transition metals and mixtures thereof.
 14. A process forpreparing a high-purity phosphor as recited in claim 11 wherein saidhigh-purity phosphor is a binary or a ternary phosphor.
 15. A processfor preparing a high-purity phosphor as recited in claim 11 wherein saidaverage precursor particle size is less than about 1 micron.
 16. Aprocess for preparing a high-purity phosphor as recited in claim 11wherein said precursor mixture is processed to obtain said sizedprecursor mixture by milling said precursor mixture in a reciprocatingmill, thereby reducing said initial average particle size of saidprecursor mixture to said average precursor particle size of said sizedprecursor mixture.
 17. A process for preparing a high-purity phosphorhaving utility as a luminescent in a field emission display, saidprocess comprising:providing a host lattice starting material selectedfrom the group consisting of silicates, aluminates, oxides, gallates,vandates, tungstates, phosphates, fluorides, oxysulfides, and mixturesthereof, and providing a dopant starting material selected from thegroup consisting of lanthanides, transition metals, and mixturesthereof; combining said host lattice starting material and said dopantstarting material in a precursor mixture having an initial averageparticle size, wherein said precursor mixture consists essentially ofsaid host lattice starting material and said dopant starting materialand is substantially free of any contaminant-generating componentscontaining a Group IA ion or a Group IIA ion; milling said precursormixture to obtain a sized precursor mixture having an average precursorparticle size less than about 1 micron; placing said sized precursormixture in a substantially impervious heat treatment vessel formed fromplatinum; and heating said sized precursor mixture in said heattreatment vessel to a temperature between about 1400° C. and 1800° C.for a time sufficient to infiltrate said dopant starting material intosaid host lattice starting material, thereby producing a high-purityphosphor.
 18. A process for preparing a high-purity phosphor as recitedin claim 17 wherein said phosphor has an average product particle sizeno greater than about 50% larger than said average precursor particlesize.
 19. A process for preparing a high-purity phosphor having utilityas a luminescent in a field emission display, said processcomprising:providing a host lattice starting material and a dopantstarting material; combining said host lattice starting material andsaid dopant starting material in a precursor mixture having an initialaverage particle size and substantially free of any fluxes orantioxidants; processing said precursor mixture to obtain a sizedprecursor mixture having an average precursor particle size less thanabout 2 microns; placing said sized precursor mixture in a heattreatment vessel formed from platinum; and heating said sized precursormixture in said heat treatment vessel to a temperature between about1200° C. and 2000° C. for a time sufficient to infiltrate said dopantstarting material into said host lattice starting material, therebyproducing a high-purity phosphor.
 20. A process for manufacturing ananode of a field emission display having a high-purity phosphor coating,said process comprising:providing a host lattice starting material and adopant starting material; combining said host lattice starting materialand said dopant starting material in a precursor mixture having aninitial average particle size and substantially free of any fluxes orantioxidants; processing said precursor mixture to obtain a sizedprecursor mixture having an average precursor particle size less thanabout 2 microns; placing said sized precursor mixture in a heattreatment vessel formed from platinum; heating said sized precursormixture in said heat treatment vessel to a temperature between about1200° C. and 2000° C. for a time sufficient to infiltrate said dopantstarting material into said host lattice starting material, therebyproducing a high-purity phosphor; and applying said high-purity phosphorto an anode screen of a field emission display.